by Cristina Marrone
Increasingly, the test takes longer to show a positive result and one of the reasons is the reactivity of our immune system which works faster than the virus
«I have Covid symptoms but the swab is negative? How come?” Many are asking the question in these weeks in which flu and Covid are galloping. Since Covid and flu have overlapping symptoms, the first possible answer is: it’s not Covid but flu. The tests that differentiate the two diseases (in addition to the syncytial virus) are only used in hospital emergency rooms and those on sale online are unreliable. So the general population can only go by exclusion. But this is not always the correct answer.
Do tampons still work?
Another doubt many people have is whether rapid home swabs also work with the new variants. Experts reassure yes, even with the new JN.1 variant. In fact, rapid antigen tests detect a protein inside the virus and not the “spike” protein on the surface of the virus which has changed with the variants. The real problem is that the test must be done at the right time.
The role of acquired immunity on test results
The sensitivity of the tests
At the beginning of the pandemic, in fact, infections occurred among people who had never caught Sars-CoV-2 before and who were not vaccinated, and the virus could act undisturbed for several days before the infection was sufficient to be detected by the immune system. Today, however, with the majority of the population vaccinated or already exposed to the virus, the immune reaction is more rapid and can lead to cases in which one has symptoms, but does not test positive, because the viral load is not yet sufficient compared to the sensitivity of the test. «If the swab is negative on the first attempt it would be advisable to repeat the test at least 24-48 hours later because it is possible that at the beginning of the symptoms there is not enough virus in the nose to detect infections» suggests Carlo Federico Perno, Professor of Clinical Microbiology , Unicamillus University.
Another reason why the rapid test can be negative is symptoms. With some variants there is less accumulation of viral cells in the nose and therefore the swab can be negative because during the sampling a sufficient quantity of biological material is not collected to result in a positive test. In some cases, the virus is more likely to be picked up in the mouth or nose than in the nostrils. Finally, the way in which the swab is performed always counts: an imprecise procedure can easily result in a false negative.
When the test line is faded
A positive result almost certainly means that the infection is present and you are contagious. False positives are quite rare. If you want to be sure you can repeat the swab with a kit from a different brand. When you test every day you may notice that the red test line goes from a faint color to a very bright color and then fades again. « If the T line appears, even if barely visible, the swab is to be read as positive and we have no indication of the viral load. With such a result the person can be contagious” clarifies Pierangelo Clerici, president of the Italian Association of Clinical Microbiologists. In rapid tests there is no weakly positive result as in molecular swabs: there is only positive or negative and by positive we mean any type of intensity or color. The intensity of the line can vary depending on how the swab is performed, how old the test is (perhaps expired), or how it was stored (it must not be kept warm) if you are recently vaccinated or infected. «It is not at all certain that a faded line corresponds to a low viral load so people must continue to implement the precautions that we know well» says Clerici.
How long are you contagious?
It is possible to spread virus particles as early as a day or two before symptoms appear. You are most contagious two or three days after the onset of symptoms until the fifth day. The available data – concludes Carlo Federico Perno, who is also responsible for Microbiology and Immunology Diagnostics at the Bambino Gesù Hospital in Rome – suggest that in general, five days after the onset of symptoms one is no longer contagious because the virus has stopped replicating and the test often measures residues of viral proteins that are not necessarily infectious.” However, the expert warns, these are data to be considered with caution: immunocompromised people and the elderly, due to their immunological situation, become infected even with very small quantities of infectious viral particles.
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January 12, 2024 (modified January 12, 2024 | 08:55)
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